Arrays

• 4.1 Use the literal syntax for array creation. eslint: no-array-constructor

  // bad
  const items = new Array();
  // good
  const items = [];

• 4.2 Use Array#push instead of direct assignment to add items to an array.

  const someStack = [];
  // bad
  someStack[someStack.length] = 'abracadabra';
  // good
  someStack.push('abracadabra');

• 4.3 Use array spreads ... to copy arrays.

  // bad
  const len = items.length;
  const itemsCopy = [];
  let i;
  for (i = 0; i < len; i += 1) {
    itemsCopy[i] = items[i];
  }
  // good
  const itemsCopy = [...items];

  
  

• 4.4 To convert an iterable object to an array, use spreads ... instead of Array.from.

  const foo = document.querySelectorAll('.foo');
  // good
  const nodes = Array.from(foo);
  // best
  const nodes = [...foo];

• 4.5 Use Array.from for converting an array-like object to an array.

  const arrLike = { 0: 'foo', 1: 'bar', 2: 'baz', length: 3 };
  // bad
  const arr = Array.prototype.slice.call(arrLike);
  // good
  const arr = Array.from(arrLike);

• 4.6 Use Array.from instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.

  // bad
  const baz = [...foo].map(bar);
  // good
  const baz = Array.from(foo, bar);

• 4.7 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following 8.2. eslint: array-callback-return

  // good
  [1, 2, 3].map((x) => {
    const y = x + 1;
    return x * y;
  });
  // good
  [1, 2, 3].map(x => x + 1);
  // bad - no returned value means `acc` becomes undefined after the first iteration
  [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
    const flatten = acc.concat(item);
    acc[index] = flatten;
  });
  // good
  [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
    const flatten = acc.concat(item);
    acc[index] = flatten;
    return flatten;
  });
  // bad
  inbox.filter((msg) => {
    const { subject, author } = msg;
    if (subject === 'Mockingbird') {
      return author === 'Harper Lee';
    } else {
      return false;
    }
  });
  // good
  inbox.filter((msg) => {
    const { subject, author } = msg;
    if (subject === 'Mockingbird') {
      return author === 'Harper Lee';
    }
    return false;
  });

• 4.8 Use line breaks after open and before close array brackets if an array has multiple lines

  // bad
  const arr = [
    [0, 1], [2, 3], [4, 5],
  ];
  const objectInArray = [{
    id: 1,
  }, {
    id: 2,
  }];
  const numberInArray = [
    1, 2,
  ];
  // good
  const arr = [[0, 1], [2, 3], [4, 5]];
  const objectInArray = [
    {
      id: 1,
    },
    {
      id: 2,
    },
  ];
  const numberInArray = [
    1,
    2,
  ];